1. Field of the Invention
The present invention relates to a direct exposure apparatus having a light source for projecting light onto an exposure target or, more specifically, an exposure target substrate, and a direct exposure method for exposing the exposure target substrate by projecting light from the light source onto the exposure target substrate.
2. Description of the Related Art
Generally, a wiring pattern on a wiring substrate is formed by exposing the substrate based on design data defining the wiring pattern and by developing and printing the desired pattern on the substrate, followed by etching. In one exposure method as an example, the exposure is performed using a photomask. In this method, a photosensitive resin is applied over a substrate fabricated by bonding metal foil as a conductive layer onto an insulating material, and a photomask with the desired wiring pattern written thereon is placed over it. By exposing the thus prepared substrate, the wiring pattern of the photomask is transferred to the photosensitive resin on the substrate. Thereafter, by performing the steps of developing, etching, plating, etc., the metal foil on the substrate is formed as the desired wiring pattern.
In the exposure method using such a photomask, the photomask is placed in intimate contact with the substrate prior to exposure; if, at this time, any foreign matter or stain is left adhering to the substrate, the shape of the foreign matter or stain is also transferred onto the substrate. The finer the wiring pattern on the substrate is, the more sensitive the photomask is to foreign matter and stains and, therefore, particularly strict management of the photomask is required. Further, if the wiring pattern is changed or damaged, a new photomask must be produced. Furthermore, the photomask must be produced on a trial basis a number of times by making corrections accounting for the expansion, shrinkage, distortion, displacement, etc. that can occur in the substrate. The resulting increase in production cost and time imposes a greater burden on the fabrication process of the wiring substrate.
On the other hand, a patterning method based on direct exposure that does not use photomasks has been proposed in recent years. According to this method, as corrections for the expansion, shrinkage, distortion, displacement, etc. of the substrate can be made in real time or in advance at the exposure data generation stage, significant improvements can be achieved, for example, in enhancing the production accuracy, increasing the production yield, shortening the delivery time, and reducing the production cost.
As one example of the patterning method based on direct exposure, Japanese Unexamined Patent Publication No. 10-112579 discloses a method that forms an exposure pattern by direct exposure using a Digital Micromirror Device (DMD). FIG. 19 is a diagram illustrating one prior art example of a direct exposure apparatus using the DMD. When directly exposing the resist formed on an exposure target substrate 3 moving relative to the DMD 51, pattern data corresponding to the pattern to be exposed is generated by a pattern generator 52, and this pattern data is input to the DMD 51. The pattern generator 52 operates in conjunction with a position sensor 53 that detects the position of the exposure target substrate 3 moving in relative fashion, and thus the pattern generator 52 generates the pattern data in a manner synchronized to the position of the exposure target substrate 3. A light source 2 projects light onto the DMD 51 through a diffusing plate 54 and a lens 55. The DMD 51 causes each of its tiny mirrors (micromirrors) to tilt according to the pattern data, thereby appropriately changing the direction of the light reflected by each micromirror on the DMD 51 and thus projecting the light through a lens 56 onto the resist on the exposure target substrate 3 to form the exposure pattern corresponding to the pattern data.
In the direct exposure method, the light source for projecting light onto the exposure target substrate must be constructed to provide uniform and evenly distributed light over the entire surface of the exposure target substrate in order to achieve a good exposure result.
FIG. 20 is a diagram showing one prior art example of the light source constructed to provide uniform illumination using a reflecting plate in the direct exposure apparatus. The reflecting plate 57 is placed behind the light source 2, and any unevenness in the light reflected from the reflecting plate 57 as well as in the light emitted directly from the light source 2 is corrected using the diffusing plate 54.
FIG. 21 is a diagram showing one prior art example which uses laser diodes for the light source in order to obtain uniform illumination in the direct exposure apparatus. The light source 2 is constructed by arranging the laser diodes 58 in a two-dimensional array, and the light emitted from the light source is projected onto the diffusing plate 54 to obtain uniform illumination.
Further, as disclosed in Japanese Unexamined Patent Publication No. 2002-367900, there is also proposed a technique that provides uniform illumination over the entire surface of the exposure target substrate by controlling the tilt angle of each micromirror on the DMD based on data concerning the distribution of the amount of image light on the exposure target substrate.
With the above-described approach that uses the reflecting plate in order to provide uniform illumination over the entire surface of the exposure target substrate, it is difficult in practice to produce an ideal surface-area light source because the light source 2 is not an ideal point light source.
FIG. 22 is a diagram showing the intensity distribution of light emission of a laser diode; as shown in FIG. 22, the intensity distribution of the light that a single laser diode emits obeys a Gaussian distribution (normal distribution). FIG. 23 is a diagram schematically illustrating the illuminance distribution of a surface-area light source constructed by arranging laser diodes in a two-dimensional array. In the figure, the darker the area, the lower the intensity of illumination. As the intensity of light emitted from a laser diode obeys a Gaussian distribution, the intensity of light emission decreases exponentially with the distance from the center of the laser diode. Usually, a laser diode is provided with a feedback circuit for controlling the intensity of light emission in order to obtain stable light output. Accordingly, in the case of the surface-area light source constructed by arranging laser diodes in a two-dimensional array, characteristics close to those of surface light emission can be obtained to a certain extent. However, as shown in FIG. 23, the illuminance decreases toward the periphery of the surface-area light source as the contributions from adjacent laser diodes decrease. That is, with the feedback circuit alone that each individual laser diode has, it is not possible to sufficiently control the illuminance distribution of the light obtained from the surface-area light source constructed from a two-dimensional array of laser diodes.
FIG. 24 is a diagram for explaining a technique (a first example) for improving the uniformity of illumination when the surface-area light source is constructed by arranging the laser diodes in a two-dimensional array. According to this technique, in the surface-area light source constructed from a two-dimensional array of laser diodes each having a uniform light output, all the laser diodes are driven to emit light, but the light emitted from the laser diodes (indicated by open circles) located in the peripheral area of the surface-area light source where the contributions from adjacent laser diodes drops is blocked by using a filter or the like (not shown) so that only the light emitted from the laser diodes (indicated by solid circles) located in the center area is used for exposure. This serves to improve the uniformity of illumination, but the efficiency drops as there are many laser diodes that do not contribute to exposure.
FIG. 25 is a diagram for explaining a technique (a second example) for improving the uniformity of illumination when the surface-area light source is constructed by arranging the laser diodes in a two-dimensional array. According to this technique, the surface-area light source is constructed by using laser diodes having different output power levels. More specifically, as shown in FIG. 25, the laser diodes located nearer to the periphery (indicated by solid shading) of the surface-area light source have larger output power, while the laser diodes located nearer to the center (indicated by stippled shading) have smaller output power. With this technique, however, once the surface-area light source has been constructed, it is difficult to make readjustments thereafter; furthermore, it is not possible to flexibly accommodate future changes in exposure conditions, etc.
On the other hand, in the case of the technique disclosed in Japanese Unexamined Patent Publication No. 2002-367900, as the tilt angle of each micromirror is controlled not only according to the amount of light falling on the surface of the exposure target substrate but also according to the pattern data generated for forming the wiring pattern, generation of data for controlling the tilt angles of the micromirrors becomes very complex.
In view of the above problems, it is an object of the present invention to provide a direct exposure apparatus and a direct exposure method wherein provisions are made so that not only can the light to be projected onto an exposure target substrate be controlled to provide the desired illuminance distribution, but also the intended illuminance distribution can be easily obtained even when exposure conditions are changed.